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Full text of "Physics 11, Snell's Law Experiment"

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Physics Experiment 6 - Snell's Law 


In this experiment you will measure the refraction of light as it passes from one medium 
to another, and you will determine the index of refraction of a given liquid. 

Equipment : 

A semicircular petri dish, a paper protractor, a light box with single slit baffle, a liquid. 


Incident angle: the angle of the incoming light (taken from the normal). 

Refracted Angle: the angle of the refracted light (taken from the normal). 
Refraction: the change of direction of light at the boundary between two media. 
Index of refraction: the ratio of the speed of light in vacuum to the speed of light in a 
medium, a number given by n = c / v 
Normal : the line perpendicular to the boundary of the medium 


The relationship between the angle of incidence and the angle of refraction was 
discovered in 1621 by the Dutch scientist Willebrod Snell (1591 - 1626). 

The relationship is known as Snell's Law which states that a ray of light bends in such a 
way that the ratio of the sine of the angle of incidence to the sine of the angle of 
refraction is in the same ratio as the velocity of light in the incident medium to the 
velocity of light in the refracted medium: 

sin 8j / sin r = Vj/v r 

The angles are taken from the normal, the line perpendicular to the boundary. 

If you gather all the incident terms on the left hand side of the equation and the refracted 
terms on the right, and using the index of refraction (n = c / v ) then Snell's law is given 

n j sin 6i = n r sin 6 r 

You can use Snell's Law to determine the index of refraction of an unknown medium by 
measuring the incident angle and the refracted angle and then solving for n r namely, 

n r = n i sin 0| / sin 9 r 


1. Read pages 350 to 353 in Physics - Principles and Problems. 

2. Copy Table 1. Incident and Refracted Angles shown below in your notebook. 

3. Fill the semicircular petri dish about three quarters full with liquid. Note the 
name of the liquid and the physical attributes of the liquid in your note book. 

Incident Angle 

Refracted Angle 





Table 1. Incident and Refracted Angles 

4. Place the flat front of the petri dish along the 90-90 line on the paper protractor. 
The normal will then be along the 00-180 line on the paper protractor. 

5. Place the baffle in the light box and turn the light box on. Align the light box so 
that the incident angle of the light ray is zero (the light ray is along the 00-180 line). 

6. Confirm that the refracted angle is zero (if not align the system so that it is zero). 

7. In increments of 10 ° change the incident angle from ° to 30 ° and measure the 
refracted angle. Record the refracted angles in your table. Estimate the uncertainty in 
the incident and refracted angles. 


1. If the index of refraction of air is taken as n , = 1.003, use the data and Snell's 
Law to calculate the index of refraction of the liquid. Calculate a value for n r for the 
angles ° to 30 ° . 

2. To the precision of the data, does the index of refraction depend on the angle? 

3. Find a mean value for the index of refraction in the medium. 

Experimental Write-up: 

Complete an experimental write-up. Your experimental write-up should include, but not 
be limited to, the purpose of the lab, all the data collected in this experiment, the table, 
and analysis outlined above, all the calculations you did to complete the lab work, as 
well as a conclusion. 

© PSCB/ May 2004 

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